The individual ionic currents of nerve cell bodies will be separated and their conduction mechanisms analyzed from the standpoint of excitability and permeability characteristics. A detailed description of the diverse conduction processes should provide insight at the molecular level on excitability and ion transport. At the functional level, a more complete description of electrophysiological properties should allow fuller understanding of the broad range of neuronal activities including action potentials, spontaneous activity and excitation-secretion coupling. Attention will be focused on the Ca current. Ca-activated K currents and the Na current. A suction pipette method will be used for voltage clamp and internal perfusion of manually isolated individual Helix neurons. The limitations and advantages of the method are described. Intracellular Ca will be measured spectrophotometrically. Macroscopic relaxation currents will be examined for steady-state and kinetic (activation and inactivation) parameters. Microscopic fluctuations of currents ("noise") will also be analyzed. Various models beginning with that of Hodgkin and Huxley will be tested with results from the two different methods. Broad-band noise will be used as a small perturbation to examine membrane impedance. The latter together with background voltage noise will provide suitable background correction for current "noise" analysis as well as a test for linearized models of ion conduction.